In the prior art, circuits are described that use nonlinear and dispersive transmission lines to produce pulsed RF signals. (As used herein, the terms radio frequency (or RF) refers to microwave or longer electromagnetic waves.) An electrical pulse is injected into such a transmission line, and the nonlinear and dispersive characteristics of the line act to form a high-frequency signal as the pulse propagates along the transmission line. Several different types of nonlinear material and several types of dispersive characteristics are available to be employed to produce RF circuits with output frequencies in the 0.1 GHz to 100 GHz frequency range.
Specifically, it is known to use nonlinear, dispersive transmission lines to convert video pulses (electrical pulses having a broad top-hat shape, with a short rise time compared with their pulsewidth, i.e. a constant amplitude for almost all of their duration) into RF pulses. A typical form of nonlinear, dispersive transmission line for this application is a variant of a standard inductance/capacitance (LC) ladder transmission line, to which nonlinearity and dispersive characteristics have been added. For example, it is known to add non-linearity to such a transmission line by providing saturable magnetic material in the inductive elements of the ladder, and to provide increased dispersion by coupling an extra capacitor across pairs of LC unit components of the ladder. Typically, the nonlinear dispersive transmission lines are created by placing thin sheets of conductors and insulators on top of each other to provide the required transmission-line components. GB 2317752A (British Aerospace) describes an alternative structure, in which the capacitative elements are formed from conductive blocks. This arrangement reduces the effects of stray capacitances and stray inductances, and thereby increases the upper limit of the generated frequency and operating RF voltage of the transmission line.
In the prior art, techniques to change the frequency and timing of the RF output pulse are described, usually by application of an adjustable DC current, voltage signal or applied magnetic field to the transmission line. GB 2 368 213A (BAE Systems plc) describes a pulse generator using a nonlinear, dispersive transmission line in which the frequency of the generated signal can be tuned over a much wider range at high powers than was the case with previous pulse generators. A nonlinear and dispersive LC ladder transmission line, of the type described above, is tuned by applying a low-power electrical signal to the nonlinear inductance components, thereby modifying the extent of the nonlinearity of the inductance component, and hence the output frequency of the generated signal. Specifically, a low-power direct current is applied to the transmission line, which flows in the inductors and couples with the saturable magnetic material in each inductor. The initial dc current is used to set the saturable magnetic material at a certain point on its hysteresis loop; the dc current can then be adjusted to tune the generated signals.
It is also known to use nonlinear capacitors in the LC circuit elements of the transmission lines. The LC circuit elements can then be tuned by varying the initial charge voltage of the nonlinear capacitors.
An ability to control the frequency and the timing of the RF output pulse from nonlinear dispersive transmission line circuits allows arrays of these circuits to be operated in phase synchronism. In this way large, RF-transmitting arrays can be produced that provide control over the output frequency and relative phase of each element in the array. U.S. Pat. No. 7,342,534 (Seddon et al.) describes a phased-array RF pulse generator in which an array of such tunable, nonlinear, dispersive transmission lines are provided. Each transmission line of the array is individually tunable to provide its generated signal with a desired phase relative to the phases of the other transmission lines of the array, enabling effects such as beam shaping and beam steering to be achieved in the overall generated signal.
Whilst each of those prior-art documents describes nonlinear, dispersive transmission lines based on LC ladder transmission lines, nonlinear, dispersive transmission lines can be achieved in other ways. For example, WO 2007/141576A1 (BAE Systems plc) describes a nonlinear, dispersive transmission line that transfers video-pulse energy to RF frequencies using a transmission line modulator based on an impulse-excited gyromagnetic action.
In order to ensure the correct output frequency and timing relationships between several individual RF sources in an array, the frequency and relative phase of the generated RF is monitored and any required adjustments are made to the system to bring it to the correct relationships. The output signal from this type of transmission line circuit is usually a short duration RF pulse with duration of a few RF cycles up to about 100 RF cycles. The short duration of the RF signal makes it difficult to determine the phase and frequency of each RF source. Known prior-art techniques include measuring the frequency of the generated RF signals by passing the RF signal through a dispersive structure, such as a RF delay line, and measuring the pulse propagation time through the RF delay line. Knowledge of the dispersion characteristics of the RF delay line (in particular the propagation velocity as a function of frequency) allows the frequency of the signal to be calculated from measurement of the propagation delay. It is conventional practice to analyse the frequency and phase of short-duration pulsed RF signals using fast transient recording devices that are based on analog to digital convertors (ADCs). The analog RF signal is sampled and digitised, and the digitised signal can then be processed to give a wide range of signal characteristics such as frequency content and relative phase information. Individual ADC devices are currently limited to sampling rates of about 4 GS/s. In order to faithfully represent signals with frequencies of 1 GHz or higher, it is necessary to use interleaving techniques, repetitive pulse sampling and sophisticated digital data processing. Those techniques require relatively expensive systems that are suitable for monitoring only relatively small numbers of RF sources. However, large transmitter arrays require a significant number of measurement channels (much more than 10 elements), for which an ADC-based control system is both bulky and costly (typically several tens of thousands of pounds).
It would be advantageous to provide a phased-array RF pulse generator in which one or more of the aforementioned disadvantages is eliminated or at least reduced.